Like what? I assume you'd do a factory reset, so really all you need to check is that all the hardware works. Make sure the screen looks good, try all the buttons, and take a shot with the camera.
–
Al E.Jan 15 '12 at 16:56

Well, I hoped that there might be an app out there that can do that testing for me...
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kfirJan 15 '12 at 19:59

4 Answers
4

Every phone that is sold by a reputable retailer
(T-Mobile, Cingular, Verizon, Sprint, etc.) should contain a tiny
indicator that clearly shows if water has attacked your phone. This
little item is typically called the liquid damage indicator, or LDI
for short. In almost every case, a red or pink indicator means that
there is water damage. A white indicator means that there is no water
damage.

The other answers cover some device specific things to check for, or fixable software issues. I'd take a different approach - check the hardware basics (screen, audio, buttons) and equally important - get on the phone with your carrier before buying to make sure the device can be activated - not blacklisted as lost/stolen, for instance.

Some Samsung Galaxy S2 units that have outdated firmwares may have yellow tints on the LCD. This is not a sign for alarm; just upgrade the firmware and everything will be fine. In case the yellow tint is still there after a firmware upgrade, consult with Samsung Service Center to have it checked or replaced. (Yellow tint: Samsung Galaxy S2 - Screen Problems)

Pink camera spots were an issue too, but seems to have been fixed by firmware upgrades. Again, check with Samsung if it cannot be fixed by firmware upgrade.

You also might want to check with Voodoo CarrierIQ detector if this device is clean. Besides, I also bought a used one and when you manage to reset it you should be fine. Type *2767*3855# for this if the password is no longer available. If your password for resetting is still available (e.g. if the previous owner knows it) there is no need for this combination obviously.